High coherent power, two-dimensional surface-emitting semiconductor diode array laser

ABSTRACT

A semiconductor laser is formed on a semiconductor substrate with an array of laterally spaced laser device elements each including a second order distributed feedback grating bounded by distributed Bragg reflector gratings. The device elements in which the distributed feedback grating and the distributed Bragg reflector gratings are formed have a lower effective index than the index of the interelement regions and are spaced so as to form an antiguided array.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of and claims priority toU.S. patent application Ser. No.10/345,613, filed on Jan. 16, 2003,pending, which claims the benefit of provisional application No.60/350,826, filed Jan. 18, 2002, the disclosures of which areincorporated by reference.

REFERENCE TO GOVERNMENT RIGHTS

This invention was made with United States government support awarded bythe following agency: NSF 0200321. The United States government hascertain fights in this invention.

FIELD OF THE INVENTION

This invention pertains generally to the field of semiconductor diodelasers and diode laser arrays.

BACKGROUND OF THE INVENTION

Phase locked arrays of antiguide laser structures were demonstrated overa decade ago. See, D. Botez, et al. “High-Power Diffraction Limited BeamOperation from Phase Locked Diode Laser Arrays of Closely Spaced ‘Leaky’Wave Guides (Antiguides),” Appl. Phys. Lett., Vol. 53, 1988, pp. 464 etseq. In an antiguide laser, the antiguide core has an index n_(o) lowerthan the cladding index, n₁. Whereas in a positive index guide, light istrapped in the guide core via total internal reflection, in an antiguidelight is only partially reflected at the antiguide-core boundaries.Light refracted into the cladding layers is radiation leaking outwardlywith lateral (projected) wavelength λ₁, and can be thought of as aradiation loss, α_(r). See, D. Botez, “Monolithic Phase-LockedSemiconductor Laser Arrays,” Chapter 1 in Diode Laser Arrays, D. Botezand D. R. Scifres, Eds. Cambridge, U.K., Cambridge Univ. Press, 1994,pp. 1-72. For a proper mode to exist, α_(r) has to be compensated for bygain in the antiguide core. The effective indices of the supported leakymodes have values below the core index, the quantum-mechanicalequivalent being quasi-bound states above a potential barrier. Althoughin a single antiguide the radiation losses can be quite high, closelyspacing antiguides in linear arrays significantly reduces the devicelosses, since radiation leakage from individual elements mainly servesthe purpose of coupling the array elements.

Due to lateral radiation, a single antiguide, for which the index ofrefraction varies in only one dimension, can be thought of as agenerator of laterally propagating traveling waves of wavelength λ₁. Inan array of antiguides, elements will resonantly couple in-phase orout-of-phase when the interelement spacings correspond to an odd or evennumber of (lateral) half-wavelengths λ₁/2, respectively. When theresonance condition is met, the interelement spacings become Fabry-Perotresonators in the resonance condition, and thus full transmission occursthrough the array structure allowing each element to equally couple toall others (i.e., global coupling is achieved). Resonant leaky-wavecoupling allows the realization of global coupling for any type of phaselocked laser array. See, D. Botez, et al. “Resonant Leaky-Wave Couplingin Linear Arrays of Antiguides,” Electron. Lett., Vol. 24, August 1988,pp. 1328-1330; D. Botez, et al. “Resonant Optical Transmission andCoupling in Phase-Locked Diode Laser Arrays of Antiguides: The ResonantOptical Waveguide Array,” AppI. Phys. Lett., Vol. 54, May 1989, pp.2183-2185. At its resonance, the in-phase mode intensity profile becomesuniform. The unwanted out-of-phase mode(s) are non-resonant, whichcauses their fields to be significantly trapped between elements, andthese modes can thus be effectively suppressed using interelement loss.Another way of suppressing out-of-phase modes is by the use ofintracavity Talbot-type spatial filters. See, D. Botez, et al.,“Phase-Locked Arrays of Antiguides: Modal Content and Discrimination,”IEEE J. Quantum Electron., Vol. 26, March 1990, pp. 482-485. However,the above description for resonant leaky-wave coupling holds only forstructures in which the index of refraction varies periodically in onlyone dimension, the lateral one. For real antiguided devices, the indexof refraction varies periodically in two dimensions, such that resonantcoupling of the elements does not necessarily occur when theinterelement spacings correspond to an odd or even number of (lateral)half-wavelengths λ₁/2, respectively, but rather when the interelementspacings correspond to an odd or even number of (lateral)half-wavelengths λ₁/2 plus a length which is a function of thetwo-dimensional details of the structure. See, D. Botez, “Monolithicphase-locked semiconductor laser arrays,” pp. 1-71 in “Diode LaserArrays,” D. Botez and D. R. Scifres eds., Cambridge Univ. Press, UK,1994.

Edge emitting devices, called ROW arrays, have exceeded the one wattcoherent-power barrier (D. Botez, et al., “Watt-Range, Coherent UniphasePower from Phase-Locked Arrays of Antiguide Diode Lasers,” Appl. Phys.Lett., Vol. 58, May 1991, pp. 2070-2072), demonstrated 10 W of peakpulse power in a beam twice the diffraction limit (H. Yang, et al., “10W Near-Diffraction-Limited Pulsed Power From 0.98 μm-Emitting, Al-FreePhase Locked Antiguided Arrays,” Electron. Lett., Vol. 33, 1997, pp.136-138), and 1.6 W continuous wave (CW) power in a twice diffractionlimited beam (H. Yang, et al., “1.6 W Continuous-Wave Coherent PowerFrom Large-Index-Step [Δn≈0.1] Near-Resonant Antiguided Diode LaserArrays” Appl. Phys. Lett., Vol. 76, 2000, pp. 1219-1221). Thesemilestones in stable, coherent power were due both to global coupling aswell as to high built-in index steps (Δn=0.05-0.10) structures, whichmakes the desired in-phase mode relatively insensitive to gain spatialhole burning (GSHB) and thermal lensing. Comprehensive above-thresholdanalyses have confirmed the basic immunity of ROW arrays to GSHB.Furthermore, unlike evanescent-wave-coupled arrays, ROW arrays do notdisplay coupling-induced instabilities, as expected for globally-coupledarrays. ROW arrays, due to large index steps as well as reliance onperiodic gain modulation for selecting lasing of specific traveling-wavemodes, were effectively the first active photonic lattices (APLs)employed for the generation of high coherent power. Bloch-functionanalysis showed them to be equivalent to 2^(nd) order complex-coupledlateral distributed feedback (DFB) structures of zero stopgap, andfurther Bloch-function analyses of finite structures have allowed thederivation of analytical formulae for all relevant design parameters.

Antiguided-array structures have also been used for creating otherAPL-type devices. These include flat phasefront, stable beam fanout MOPAdevices (See, Zmudzinski, et al., “3-Core ARROW-Type Diode Laser: NovelHigh-Power Single-Mode Device, and Effective Master Oscillator forFlared Antiguided MOPAs,” IEEE J. Select. Topics Quantum Electron., Vol.1, No. 2, June 1995, pp. 129-137; D. Botez, et al., “Flat PhasefrontFanout-Type Power Amplifier Employing Resonant-Optical WaveguideStructures,” Appl. Phys. Lett., Vol. 63, December 1993, pp. 3113-3115),ARROW devices (L. J. Mawst, et al., “Design Optimization of ARROW-TypeDiode Lasers,” IEEE Photonics Tech. Lett., Vol. 4, November 1992, pp.1204-1206; L. J. Mawst, et al., “High-Powered, Single Mode, Al FreeInGaAs, [P]/InGaP/GaAs Distributed Feedback Diode Lasers,” Journal ofCrystal Growth, Vol. 195, 1998, pp. 609 et seq.; D. Zhou, et al.,“Simplified Antiresonant Reflecting Optical Wave Guide-TypeVertical-Cavity Surface-Emitting Lasers,” Appl. Phys. Lett., Vol. 76,2000, pp. 1659 et seq.); and Triple-Core ARROW Devices (A. Bhattacharya,et al., “0.4 W CW Diffraction-Limited-Beam Al-Free, 0.98 μm Three CoreARROW-Type Diode Lasers,” Electron. Lett., Vol. 32, 1996, pp. 657-658)which have demonstrated high CW (≧0.4 W) coherent powers, as well aswell as one-dimensional and two-dimensional ROW arrays of verticalcavity surface emitting lasers (VCSELs) (S. K. Serkland, et al.,“Two-Element Phased Array of Antiguided Vertical-Cavity Lasers,” Appl.Phys. Lett., Vol. 75, 1999, pp. 3754 et seq.; D. Zhou, et al.,“Two-Dimensional Phase-Locked Antiguided Vertical CavitySurface-Emitting Laser Arrays,” Appl. Phys. Lett., Vol. 77, 2000, pp.2307 et seq.). However, ROW arrays can be prone to self-pulsationseither if saturable absorption occurs in lossy interelement regions orwhen imaging, in intracavity Talbot-type spatial filters, is disturbedby GSHB. Single-frequency pulsed operation can be achieved foredge-emitting devices by using DFB gratings, but the yield ofin-phase-mode operating devices has been quite low, since the effectiveyield is a strong function of the grating phase(s) with respect to thecleaved mirror facet(s). M. P. Nesnidal, et al., “Distributed FeedbackGrating Used as an Array-Mode Selector in Resonant Antiguided DiodeLaser Arrays: Effects of the Mirror Facet Position With Respect to theGrating,” IEEE Photon. Tech. Lett., Vol. 10, 1998, pp. 507 et seq.; andN. Nesnidal, et al., “0.45 W Diffraction-Limited-Beam andSingle-Frequency Operation from Resonant Antiguided Phase-Locked LaserArray With Distributed Feedback Gratings,” Appl. Phys. Lett., Vol. 73,1998, pp. 587 et seq.

Second-order DFB laser structures for use as surface emitters, based onoutcoupling perpendicular to the chip surface, have been studied fornearly three decades. However, it has been found both theoretically aswell as experimentally that the favored mode to lase is an antisymmetricone (that is, a two-lobed pattern), since it has the least radiationloss. Furthermore, the guided-field pattern is highly nonuniform, makingthe device vulnerable to multimoding via longitudinal GSHB. C. H. Henry,et al., “Observation of Destructive Interference in the Radiation Lossof Second-Order Distributed Feedback Lasers,” IEEE J. QE, Vol. 21, 1985,pp. 151-153.

Several approaches have been tried to obtain symmetric-like modeoperation or actual symmetric mode operation. The first approachinvolves using a π phase-shifting film deposited on half the deviceaperture (S. H. Macomber, et al., “Recent Developments in SurfaceEmitting Distributed Feedback Arrays,” Proc. SPIE, Vol. 1219, 1990, pp.228 et seq.). The second approach involves a long (about 2 mm) chirpedgrating (S. H. Macomber, “Nonlinear Analysis of Surface-EmittingDistributed Feedback Lasers,” IEEE J. QE, Vol. 26, 1990, pp. 2065-2074),which phase shifts the antisymmetric mode such that the devices operatein an off-normal single lobe pattern. A third approach causes puresymmetric-mode operation either by preferential carrier injection in aweak-coupling grating region (N. W. Carlson, “Mode Discrimination inDistributed Feedback Grating Surface Emitting Lasers Containing a BuriedSecond Order Grating,” IEEE J. QE, Vol. 27, 1991, pp. 1746-1752), or byintroducing a metal grating which suppresses antisymmetric-mode lasing(M. Kasraian, et al., “Metal Grating Outcoupled, Surface-EmittingDistributed Feedback Diode Lasers,” Appl. Phys. Lett., Vol. 69, 1996,pp. 2795-2797). However, preferential carrier injection is not along-term reliable approach, and the scheme, due to the necessity forweak coupling grating, inherently leads to inefficient devices (about10% efficiency). The metal-grating variant of the approach is feasiblebut introduces too much of a penalty loss for the symmetric mode, suchthat efficiencies are at best 25-30%, and the gain thresholds are quitehigh (about 70 cm⁻¹).

More recently, a solution to obtaining a symmetric-mode beam patternwith no penalty in device efficiency has been found in the use ofcentral grating phase shifts of around π in distributedfeedback/distributed Bragg reflector (DFB/DBR) devices. G. Witjaksono,et al., “Surface-Emitting Single Lobe Operation from 2^(nd)-OrderDistributed-Reflector Lasers With Central Grating Phase Shift,” Appl.Phys. Lett., Vol. 78, 2001, pp. 4088-4090; Dan Botez, et al., “SingleMode, Single Lobe Surface Emitting Distributed Feedback SemiconductorLaser,” Published International Application No. WO 01/13480 A1, 22 Feb.2001. An example is a structure having a double-quantum-well (DQW)InGaAs/InGaAsP active region with InGaP cladding layers, and a gratingformed in a P⁺-GaAs cap layer. The DQW active region is designed to be0.4-0.5 μm away from the metal contact such that the device efficiencyand reliability are unaffected. A symmetric-like mode is favored to laseover the antisymmetric-like mode when the grating phase shift, Δφ,ranges from 100° to 280°, with maximum discrimination occurring whenΔφ=180°, i.e., a half wave (λ/2) central phase shift. The 180° phaseshift does not affect the in-plane propagating (guided) light, as thefield round trip through the phase shifter is 360° (i.e., the guidedfield remains antisymmetric). For the same reason, the 180° phase shiftregion does not affect the DFB/DBR grating, since the lasing occurs atthe same wavelength, close to the Bragg wavelength, with or without a180° phase shift. That is, the 180° phase shift creates no defect in theactive photonic lattice. However, for the grating-outcoupled light, the180° central phase shift region defines two surface emitting regionswhose outcoupled fields are out-of-phase with each other, and thus theoutcoupling of the guided antisymmetric field provides in-phase (i.e.,symmetric) radiated near-field and far-field patterns. These types ofdevices also allow for relatively large tolerances in devicefabrication, providing a practical solution for single(orthonormal)-lobe efficient surface emission from 2^(nd)-order DFBlasers.

For devices optimized for maximum external differential quantumefficiency, η_(d), the variation of the threshold gain and η_(d) havebeen studied as a function of the grating duty cycle, σ, defined as theratio of Au as part of the grating period. G. Witjaksono, et al.,“High-Efficiency, Single-Lobe Surface Emitting DFB/DBR Lasers,” PaperTuA3, 14^(th) IEEE LEOS. Annual Meeting, 12-15 Nov. 2001, San Diego,Calif. The intermodal discrimination, Δα, reaches a maximum 113 cm⁻¹ forσ=0.5, while the symmetric mode (S-mode) threshold gain is only 22 cm-⁻¹for σ=0.4, with a respectable Δα value of 52 cm⁻¹. In general, it isfound that such devices can tolerate some variation in grating dutycycle at a relatively small penalty in slope efficiency.

Gratings with phase shifts can be patterned by e-beam lithography or byholographic exposure of side-by-side negative and positive resists.However, current e-beam lithography allows writing of gratings only400-600 μm long, and for devices requiring relatively long gratings(e.g., about 1,500 μm), fabrication by e-beam lithography is notadvisable. The holographic method has been used to fabricate1^(st)-order gratings with quarter-wave (i.e., π/2) phase shifts, withthe transition from negative to positive resists creating a gratingphase shift of half the grating period. Using the same method for2^(nd)-order gratings naturally provides half-wave (i.e., π) phaseshifts. Semiconductor (GaAs) gratings with π phase shifts have beendeveloped using negative and positive resists (G. Witjaksono, et al.paper, TuA3, supra). A transition region is observed, but its width isnot that relevant as long as the two grating regions are out-of-phasewith each other. That is, the grating phase shift does not necessarilyhave to be π; it can be an odd number of π, since the in-planepropagating (guided) light is unaffected by it.

Two-dimensional (2-D) single-mode, single-lobe surface emitters(horizontal resonant cavity) are ideal high-power (≧1 W) coherentsources due both to low aspect ratio beams as well as the potential forscaling up the power by the use of coherent coupling of the sources atthe wafer level (i.e., monolithically). L. J. Mawst, et al., “2-DCoherent Surface-Emitting Leaky Wave Coupled Laser Arrays,” IEEE J.Quantum Electron, Vol. 29, 1993, pp. 1906-1917. Three such types ofdevices have been reported. One involves angled gratings, K. N. Dzurko,et al., “Distributed Bragg Reflector Ring Oscillators: Large ApertureSource of High Single Mode Optical Power,” IEEE J. Quantum Electron.,Vol. 29, 1993, pp. 1895-1899; M. Fallahi, et al., “Low Threshold CWOperation of Circular-Grating Surface-Emitting DBR Lasers Using MQW anda Self-Aligned Process,” IEEE Photon. Tech. Lett., Vol. 6, 1994, pp.1280-1282. The third approach uses a curved-grating unstable resonator,S. H. Macomber, et al., “Curved-Grating Surface-Emitting DFB Lasers andArrays,” Proceedings Society of Photo-Optical InstrumentationEngineers,” Vol. 3001, 1997, pp. 42-54. However, none of these deviceshave a built-in dielectric structure for lateral-optical-mode controland stability, and as a result are vulnerable to temperature and carrierinduced dielectric-constant variations. An example of such behavior isthe unstable resonator device which, while operating single-mode to highpeak pulsed power in a single off-normal beam, readily becomes multimodein CW operation due to thermal lensing.

SUMMARY OF THE INVENTION

The semiconductor diode array lasers of the present invention utilizeperiodic dielectric structures with modulated optical gain, so-calledactive photonic lattices (APLs), to realize watt-range coherent,surface-emitted powers from 2-dimensional (2-D) horizontal cavitydevices with 2^(nd) order gratings of novel design. In contrast toconventional APLs, the devices in accordance with the invention havegain in the low-index lattice sites, enabling long range (coherent)coupling by traveling waves utilizing resonant leaky-wave couplingbetween the low-index lattice sites, which had previously only beenpossible for 1-dimensional edge-emitting structures (so-calledantiguided array structures).

The antiguided array structures of the present invention for the lateraldimension preferably have large index steps (e.g., Δn≈0.10), whichensure optical-mode stability against carrier and thermal induceddielectric-constant variations. The 2-D devices of the present inventioncombine antiguided phase locked arrays with surface emission from2nd-order DFB/DBR grating structures. Unlike prior 2-D 2^(nd) ordergrating DFB/DBR surface emitters, the present invention may utilizegrating phase shifts around 180°, via a discrete central phase shift ora phase shift distributed along the length of the grating (e.g., achirped grating), or a π-phase shifting surface film, to provideemission in a single lobe beam at no penalty in device efficiency. Thegrating structure, beside ensuring single longitudinal-mode operation,can be formed to act as a highly efficient selector of a single lateralmode, the in phase array mode. As a consequence, large aperture (e.g.,200 μm×1200 μm to 400 μm×1 cm) coherent laser diode sources inaccordance with the invention have nearly uniform 2-D guided-fieldprofiles and thus are able to operate in a stable, singlediffraction-limited beam to watt-range CW output powers. Beamcircularization can then be readily obtained utilizing commerciallyavailable optical components.

The present invention enables CW watt-range, stable, single mode laserlight sources that may be utilized for applications such asroom-temperature CW mid-IR (λ=3-5 μm) coherent light generation (viafrequency up-conversion) that can provide several orders of magnitudeincreases in the sensitivity of laser-absorption spectroscopy for a widearray of non-invasive medical diagnostics (e.g., breath analysis), andfor IR countermeasures; for the generation of hundreds of mWs of bluelight via harmonic conversion for applications such as in biotechnology(e.g., flow cytometry) and for laser projection systems; for high-powerlow-noise, high-fidelity RF optical links; and for coherent free-spaceoptical communications. The devices of the present invention are singlefrequency and thus ideally suited for scalability at the wafer level(via resonant leaky-wave coupling) to 20 units or more. This enablesall-monolithic laser light sources capable of providing tens of watts ofcoherent, uniphase power for various applications, such ashigh-efficiency, high resolution magnetic resonance imaging with noblegases.

The surface-emitting devices of the present invention have theadvantages over edge-emitting devices for the generation of high(greater than 1 W) CW coherent powers that: complete passivation of theemitting area is not needed for reliable operation, scalability at thewafer level becomes possible, and packaging is significantly simplified.

The semiconductor diodes of the present invention preferably utilizegain in the low-index lattice sites, which in turn allows long rangecoherent coupling via traveling waves. An effective π phase shift may beimplemented to provide single-lobe operation. For example, the use of aπ phase shifter centrally located in a second order grating serves toprovide single-lobe, orthonormal beam emission with high efficiency(greater than 60%), and the use of gratings with dual spatial-modeselectivity provides both conventional longitudinal-mode selection andstrong lateral-mode selection for phase-locked antiguided arrays. Agrating with a continuously varying phase shift along the length of thegrating may also be utilized to achieve single lobe operation. In thelateral direction, spatial coherence is obtained by using a phase-lockedantiguided array, which corresponds to a 1-dimensional active photoniclattice, and using traveling waves for element coupling. The coupling isresonant and long-range by having the high-index regions correspond toan integral number of lateral half waves, thus making the structurefully transmissive. Although the index step may be relatively smallcompared to those commonly used in photonic lattices, photonic bands andband gaps are formed. Such devices are in effect lateral 2^(nd)-ordercomplex coupled DFB structures whose 2^(nd) diffraction order provideslateral coupling while the 1^(st) diffraction order provides propagationin a direction parallel to the array elements and edge emission for edgeemitting devices. At lateral resonances, the second order Braggcondition is exactly satisfied, and the stopgaps disappear, i.e., atresonance, full transmission across the photonic lattice is allowed,effectively making such devices photonic band pass devices. Atresonance, antiguided arrays become pure gain-coupled DFBs, which arewell known to have no stopgaps and thus will lase at the Braggfrequency. Strong light absorption in the interelement regions providesdiscrimination of modes with significant interelement field, thusfavoring the in-phase mode.

In the longitudinal direction, coherence is obtained by using a2^(nd)-order grating with a DFB region for gain, partial feedback andoutcoupling, and DBR regions for strong frequency-selective feedback andpartial outcoupling, effectively providing a second order photonic bandgap structure with light outcoupling. Unlike microcavity PBG structureswith built-in lattice defect (defect lasers), the photonic band gapstructure in the present invention does not have lattice defects, thusallowing for full transmission of light over large distances and thuspermitting the generation and surface emission of high coherent powerfrom large apertures. Thus, the present invention combines 2^(nd)-orderphotonic lattices in both the lateral (PBP structure) and thelongitudinal (PBG structure) directions. The two lattices areinterconnected in that the longitudinally-placed gratings act as ahighly effective selector of the desired traveling-wave mode in thelateral direction—the in-phase array mode.

Further objects, features and advantages of the invention will beapparent from the following detailed description when taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a simplified perspective view of a surface-emitting DFB/DBRdevice operating in a single, orthonormal beam that may be utilized asan array element in the semiconductor diode laser of the presentinvention.

FIG. 2 is a schematic cross-sectional view of an exemplary semiconductorlaser element of the type shown in FIG. 1 that has a sinusoidal-shapedgrating.

FIG. 3 is a computed graph of the mode discrimination for the device ofFIG. 2 as a function of the central phase shift value, Δφ.

FIG. 4 is a computed graph for the device of FIG. 2 with Δφ=180° showingthe near-field profile in the solid curve and the guided-field profilein the dashed curve.

FIG. 5 is a computed graph illustrating the far-field pattern for thedevice of FIGS. 2 and 4.

FIG. 6 are computed graphs for the device of FIGS. 2 and 4 showing thedifferential quantum efficiency, η_(d) and the effective η_(d).

FIG. 7 is a computed graph for the device of FIGS. 2 and 4 illustratingthe guided-field peak-to-valley ratio R′ in the active (i.e., DFB)region as a function of the phase shift Δφ.

FIG. 8 is a simplified perspective view of an antiguided diode arraylaser in accordance with the invention having a central phase shiftDFB/DBR grating structure.

FIG. 9 is a perspective view of the device of FIG. 8 illustratingemission from the surface of the substrate of the device.

FIG. 10 is an illustrative cross-sectional view of the device of FIG. 9taken generally along the lines 10-10 in FIG. 9.

FIG. 11 is a graph illustrating the transverse field-intensity profilein the array element regions (n_(err)=3.28) of the device of FIGS. 8 and9.

FIG. 12 is a graph illustrating the transverse field-intensity profilein the interelement regions (n_(eff)=3.34) in the device of FIGS. 8 and9.

FIG. 13 is a graph illustrating the near-field intensity profile of theupper adjacent array mode of a 10-element ROW array.

FIG. 14 is a plan view of a laser diode array structure in accordancewith the invention that is arranged for suppressing oscillation of theupper-adjacent mode of 40-element ROW arrays.

FIG. 15 is a simplified plan view of a scaled array device in accordancewith the invention in which individual devices are coupled in the array.

FIG. 16 is an array as in FIG. 15 having 16 laser diode array units.

FIG. 17 is a simplified cross-sectional view through the interdeviceregions of the array of FIG. 16.

FIG. 18 is a schematic cross-sectional view of another exemplarysemiconductor laser element of the type shown in FIG. 1 having a gratingwith a slowly varying period along the length of the grating.

FIG. 19 is a cross-sectional view of an interelement region that may beutilized in the laser diode array of the invention to provide absorptionand discrimination against modes with significant interelement field.

FIG. 20 are graphs illustrating the results of simulations of thresholdcurrent densities for various modes in a 20-element laser diode array inwhich there is no loss in the interelement regions.

FIG. 21 is a graph showing results of simulations as in FIG. 20 withinterelement absorption provided by heavily doped GaAs.

FIG. 22 is a graph showing the results of simulations as in FIG. 20 withthe inclusion of an absorbing layer of InGaAs in the interelementregions.

FIG. 23 are graphs showing the results of simulations as in FIG. 20 forinterelement regions having both an absorbing layer of InGaAs andheavily doped GaAs in the interelement regions.

FIG. 24 is a schematic cross-sectional view of another exemplarysemiconductor laser element of the type shown in FIG. 1 with a π phaseshift layer formed on the output face of the substrate at which thelight exits to provide c-phase shift and substantially single lobeoutput.

DETAILED DESCRIPTION OF THE INVENTION

For purposes of illustrating the invention, a single surface-emittingDFB/DBR device element that may be utilized in the laser diode array ofthe invention is shown generally at 20 in FIG. 1. This type of deviceincludes a distributed feedback (DFB) grating structure 22 boundedlongitudinally by two distributed Bragg reflectors (DBRs) 24. Operationin a single orthonormal beam (illustrated at 25) can be attainedutilizing at or near a half-wave (π) central grating phase shift in theDFB grating 22. A particular embodiment of a semiconductor laserstructure of this type is shown for exemplification in FIG. 2, althoughit is understood that other devices of this type may also be utilized,as described, for example, in published International Application WO01/13480 and U.S. Pat. No. 6,810,053. As discussed further below, singlebeam operation can also be obtained utilizing a grating with adistributed phase shift or a π-phase shift film on the output surface ofa device whose grating has no phaseshift.

The illustrative structure shown in FIG. 2 has a double-quantum-wellInGaAs/InGaAsP active region 31 with InGaP cladding layers 32 and 33, aGaAs substrate 34, a GaAs cap layer 36, and a grating 37 formed ofGaAs/Au by etching in the p⁺-GaAs cap layer 36 followed by a depositionof gold shown as a layer 40, which provides both high couplingcoefficient (κ) as well as ensuring that all first-order diffractedlight is collected. The layer 40 also serves as the metal contactelectrode on the device p side. Light exits from anantireflective-coated surface 41 of the substrate 34 which is exposed byan opening 42 in a bottom electrode 43. Modeling was carried out withthe grating 37 assumed to have a sinusoidal shape as illustrated in FIG.2. The modeled device included silicon dioxide insulating layers 44 inthe DBR regions 24 that serve to direct current to the region of thedistributed feedback grating 22 to provide optical gain in this region.The edge terminations were assumed to be totally antireflective (zeroreflectivity). As shown in FIG. 3, the modeling found that as thecentral phase shift, Δφ, varies between 100° and 280°, the symmetricmode (S) is favored over the antisymmetric mode (A), with maximumdiscrimination occurring at Δφ=180°. For analysis, the model used boththe coupled-mode theory as well as the transfer-matrix method. Thecarrier-induced depression is taken into account, and the couplingcoefficients are κ_(DFB)=−5.46+i49.42 and κ_(DBR)=2.75+i30.46 fordevices with 0.3 μm-thick p-InGaP cladding and a 0.1 μm-thick InGaAsPupper confinement layer. The background absorption coefficient in theDBR regions is taken to be 15 cm⁻¹, in agreement with prior experimentalresults from InGaAs/GaAs DBR devices. An inspection of the differentialequations for the right-going, R, and left-going, S, fields as well asof the expression for the (surface-emitted) near-field reveals that aphase shift value of 180° is equivalent to placing a π phase-shift filmon half of the emitting area; that is, a 180° central phase shiftaffects only the grating-outcoupled radiation.

For devices with 500 μm-long DFB and DBR regions, FIG. 4 shows theresults of the modeling with Δφ=180°, showing the near-field (solidcurve) and guided-field (dashed curve) profiles. FIG. 5 shows thefar-field profile for Δφ=180°. The guided-field peak-to-valley ratio,R′, in the active (i.e., DFB) region is only 2, which should insuresingle-mode operation to high power since the mode discrimination ishigh (≧100 cm⁻¹) as shown in FIG. 3. The far-field consists of anorthonormal beam with 88% of the light in the central lobe.

At Δφ=180°, the differential quantum efficiency, η_(D), is 51%, whichreduces to 45% (i.e., effective η_(D)) when taking into account thecentral-lobe energy content. Both η_(D) and the effective η_(D) areplotted as a function of phase shift Δφ in FIG. 6. As for theguided-field, the peak-to-valley ratio, R′, while reaching a maximum of2 at Δφ=180°, is seen to decrease to values as small as 1.3 at Δφ=120°and 270°, as shown in FIG. 7. As seen from FIGS. 6 and 7, over a widerange in Δφ—in the range of 60°, or within 30° of 180°—the effectiveη_(D) is relatively high (≧42%) and the degree of guided-fielduniformity is low (R′<2). These devices are thus capable of providinghigh single-mode surface emitted power, e.g., >100 mW CW fromsingle-element (e.g., ridge-guide) devices, and >1 W CW from 2-Dsurface-emitting devices with parallel-coupled phased arrays in thelateral dimension.

With reference to FIG. 8, a semiconductor array laser device inaccordance with the invention is shown generally at 50 and employs aDFB/DBR structure (in the longitudinal direction) for providing feedbackand efficient coupling. Each element of the array in the lateraldirection may comprise, for example, the DFB/DBR structure 20 of FIGS. 1and 2 laterally spaced and separated by interelement regions 51. An etchstop layer 53 of GaAs is provided to divide the p-InGaP layer 32 toallow etching down to the stop layer 53 in the interelement regions 51and regrowth with p-GaAs to form interelement regions 55 separating thegratings in adjacent elements 20. Using SiO₂ as a mask for etch andregrowth, the 1 μm-wide GaAs regions 55 are periodically introduced inthe lateral direction to form high (effective) index interelementregions 51 for an antiguided array. The thin GaAs stop-etch layer 53 isused for controlling the depth of the periodic etching. The GaAs regions55 preferably have a varying p-type doping concentration: a mediumdoping concentration (10¹⁷ to 10¹⁸ cm⁻³) from the stop-etch layer towithin 0.05-0.10 μm from the region's top surface. The rest, a 0.05-0.10μm-thick layer, is heavily doped (10¹⁹ to 10²⁰ cm⁻³) with Zn or C toform a cap layer for good electrical contact to the metal electrode.After regrowth of the GaAs regions 55, made easy by the use of Al-free(i.e., InGaP) cladding-layer material 32, the SiO₂ stripes are removed,and the n-side of the 2-D source is metallized as shown in FIG. 9 toprovide a metal electrode 43 so that current may be applied across thedevice between the electrode 43 and the gold electrode layer 40. Thegold electrode layer 40 makes electrical contact with the semiconductoronly over a longitudinally restricted region, which constitutes the DFBregion of the laser (as shown in FIGS. 2 and 9). Outside of that regionthe gold layer covers insulating SiO₂ layers, which are above the DBRregions of the laser, as seen in FIG. 9, to direct current to theregions of the distributed feedback grating. Any other suitablestructures for directing current to the DFB grating regions (e.g.,back-biased junctions, etc.) may be used. To insure good adhesion of themetal electrode to the SiO₂ layers, about 20 Å of Ti and about 20 Å ofPt may be placed by evaporation and liftoff, prior to the Au deposition,on the SiO₂ layers. The output light is emitted through an AR-coatedcontact-stripe surface of the surface of the substrate defined by awindow opening 42 in the electrode 43 on the substrate n-side as shownin FIG. 9. The positions of the optical modes are illustrated in thecross-sectional view of FIG. 10.

To completely suppress reflections from the cleaved chip ends, absorbingmaterial (InGaAs) can be introduced (via etch and regrowth) at theDBR-reflector ends. This prevents disturbance of lasing in the DFB(active) region due to back reflections with random phase from theDBR-reflector ends. Even though the guided field is relatively small atthe DBR ends, random-phase reflections can propagate through the mostlybleached DBR reflectors (the mode absorption coefficient is only ≈15cm⁻¹ when considering a bulk absorption coefficient of 200 cm⁻¹) andaffect lasing in the DFB region.

Calculations were performed for a structure incorporating the gratingand GaAs regrowths of various thicknesses. FIGS. 11 and 12 show resultsfor a device with 0.28 μm-thick GaAs regrowths 55. The transverse fieldintensity profiles in the array element regions 20 [low effective index(3.28)] and interelement regions 51 [high effective index (3.34)] areshown in FIGS. 11 and 12. The index step is high (i.e. 0.06), thusinsuring stability against thermal-and carrier induceddielectric-constant variations; and corresponds to the resonancecondition for a structure of 1 μm-wide interelement spacings (width ofthe interelement regions 51).

An important feature of the array device of the invention is that thegratings provide feedback only in the array element regions 20. Thathappens because, after the creation of the high-index (GaAs) regions 55,the gratings remain only in the low-index element regions 20. Then thein-phase resonant mode, which has most of its field (≈95%) in theelement regions 20, is favored to lase over the out-of-phase mode, whichhas significant interelement field. That is, the grating has a triplerole: a) feedback; b) outcoupling, and c) selecting operation in thein-phase (lateral) array mode. This array-mode selection mechanismsubstantially eliminates the need for interelement loss and/orTalbot-type spatial filters to suppress out-of-phase mode operation,thus avoiding self-pulsations due to saturable absorption and GSHB,respectively. The use of preferential feedback as the array-modeselector for 20-element edge-emitting devices has previously beendemonstrated by using a continuing grating placed below the activeregion. See M. P. Nesnidal, et al., IEEE Photon. Tech. Lett., supra. Theyield for such devices was low because, in edge emitting devices, theintermodal discrimination is a strong function of the grating phase(s)with respect to the cleaved mirror facet(s).

In the present invention, the mode selection is strong (the gratingexists only in the element regions) and there are no grating phaseeffects, since the reflectors are of the DBR type. Therefore, the yieldof in-phase-mode array operation for the devices of the presentinvention can be quite high.

Further discrimination against oscillation of modes with significantinterelement field, such as the out-of-phase mode, is achieved via astrong light absorption in the heavily doped (10¹⁹ to 10²⁰ cm⁻³) topparts of the p-GaAs regions 55. Unlike conventional antiguided arrays,such absorption cannot be saturated since carriers created due toabsorption quickly transfer to the metal electrode.

20-element ROW arrays (≈100 μm aperture) easily operate in a singlearray mode, since there is effective discrimination against theout-of-phase mode as well as against the closest array mode to thein-phase mode: the so-called upper-adjacent mode. For the latter, onerelies on higher edge radiation losses for the upper-adjacent mode thanfor the in-phase mode. In order to obtain more coherent power as well asa relatively low beam aspect ratio (for 2-D devices) amenable to easybeam circularization with off-the-shelf optical components, a 200 μmaperture (i.e., 40-element arrays) is desirable. However, the array edgeradiation losses scale inversely proportional with the number ofelements, with the result that 40-element devices usually operatesimultaneously in two modes: the in-phase and upper-adjacent ones, thusproviding beams with lobe widths twice the diffraction limit.

The grating of the present device suppresses the upper-adjacent modeonly at or very close to perfect resonance, where the mode hassignificant interelement field. However, at the point of maximumdiscrimination via edge losses (still quite close to the in-phase moderesonance) the upper-adjacent mode has negligible interelement field andit is peaked at the array edges as shown in FIG. 13. To insure itssuppression, it is possible, for devices with a large number of elementssuch as 40-element devices, to taper the DBR-grating regions 24 in thelongitudinal direction as shown in FIG. 14, such that the upper-adjacentmode “sees” much less reflection from the DBR sections than the in-phasemode. To insure no back reflections from InGaAs absorbing regions 60introduced at the DBR-region ends, the edges 62 of the absorbing regions60 may be angled, e.g., at 45° in the tapered-grating regions, as shownin FIG. 14. Thus, light transmitted through the DBR at various pointsalong the taper will be deflected, insuring that no back reflections canupset the device operation. Thus, because the gratings are formed onlyin the element regions 20, and with the DBR-reflector taperingsuppressing the upper-adjacent mode pure diffraction-limited-beamoperation from many element (e.g., 40-element) devices is obtained.

Another feature of the invention, due to the surface-emitting nature ofthe device, is that only the fields in the array element regions 20 areoutcoupled. Thus, for the in-phase resonant mode, only in-phase fieldsare outcoupled, which in turn provides higher central-lobe energycontent (78%) in the far-field pattern than in the case of anedge-emitting array of similar geometry (63%). Thus, the overall 2-Deffective quantum efficiency (i.e., η_(D) multiplied by the percentageof energy radiated in the single orthonormal lobe) can reach values ashigh as 42%.

Exemplary devices may utilize 20-element arrays with 4 μm-wide elements20 and 1 μm wide interelements 51, thus providing a 100 μm-wide lateraldimension for the 2-D source. Longitudinally, the structure shown inFIG. 2 (e.g., with ≈1300 μm equivalent aperture) can be used tooutcouple the light. For more power and better beam aspect ratio,40-element devices (i.e., 200 μm-wide array) may be used, together with1200 μm-long DFB/DBR gratings. Then the beam aspect ratio is only 6,which allows for easy beam circularization with commercially availableanamorphic prism pairs.

For an antiguided array, radiation leaks at its edges by refraction. Inturn, 2-D surface emitters can be coupled via leaky waves indiamond-shaped 2-D configurations across the wafer surface. Since ROWarrays leak radiation laterally at predetermined angles (8-10° for anindex step of (3-5)×10⁻²), a diamond-shaped configuration for which fourROW arrays 50 are mutually coupled can be formed as shown in FIG. 15.The laser devices may have tapered DBR-grating regions and lightabsorbing layers outside the DBRs as shown in FIG. 14. Each of the fourROW arrays has its own electrode, like the one shown in FIG. 8, makingelectrical contact to their respective DFB regions. Radiation isoutcoupled through the substrate via the DFB/DBR gratings. Interdeviceelectrodes 70 can be provided, as shown in FIG. 16, that ensure, viacarrier-induced changes in the dielectric constant, that adjacent unitsare resonantly coupled. In the interdevice regions, the grown structurehas the same structural composition as that for the interelement regionsof ROW arrays (see cross-section in FIG. 12) as shown in FIG. 17. Inorder to provide independent current injection of the ROW arrays and theinterdevice electrodes 70, narrow (˜3 μm wide) electrical-isolationtrenches may be etched through the upper p-cladding layers. In the caseof array devices with tapered DBR regions, as shown in FIG. 14, the samesteps are taken except that the DFB regions extend laterally only to thearray edges. Leaky-wave coupling of ROW arrays over large distances(90-176 μm) has previously been demonstrated in both linearconfigurations as well as 2-D configurations. The 2-D configurationsinvolved Fabry-Perot lasing cavities (defined by micromachined mirrorfacets) and light-deflection in a direction normal to the chip surfacevia 45° micromachined turning mirrors. Micromachined diode-laser mirrorfacets were used only for concept-proving purposes. Otherwise, using 45°micromachined turning mirrors is a surface emitting method which doesnot provide any phase control, since micromachining cannot provide beamdeflectors equally spaced, within a fraction of a wavelength, from thelaser's emitting facets. Furthermore, because the ROW arrays had lasingcavities of the Fabry-Perot type, the devices' individual frequencyspectra were multimode, which in turn led to a rather poor degree ofoverall coherence, 35% fringe visibility, for the 2-D configuration.

Using the 4-unit diamond-shaped configuration of FIG. 15 as a buildingblock it is possible to build larger 2-D arrays (e.g., 16 units as shownin FIG. 16) all phase-locked via resonant leaky-wave coupling, andmutually frequency-locked sources operating at the same frequency. Thus,full coherence, 100% fringe visibility, can be achieved, in contrast tothe previously demonstrated 2-D configurations involving phase-lockingof array units having Fabry-Perot cavities. For 16-unit devices, thefact that most array units resonantly couple to three or four nearestneighbors is very much like the global-coupling mechanism of individualantiguided elements in ROW arrays. As opposed to previous 2-D arrayschemes, which could at best achieve 150 mW of diffraction-limiteduniphase power, the 2-D ROW-DFB array of the invention has threesignificant advantages: (1) it provides phase locking in addition tofrequency locking, (2) it represents a global-coupled 2-D monolithicarray; and (3) the interunit coupling is independent of the feedbackand/or beam-outcoupling mechanisms.

Scaling at the wafer level can be extended to at least a 25-unit 2-Darray. However, due to unavoidable layer thickness and/or compositionnonuniformities across the array, full coherence may be difficult tomaintain when the 2-D array has more than approximately 25 units. Toprovide full coherence in arrays with large numbers of elements, asingle frequency master oscillator (MO) may be utilized. The MO can be asingle-frequency laser monolithically integrated on the wafer or anexternal single-frequency laser. In either case, an optical isolatorshould be provided between the MO and the 2-D array to insure that noback reflections affect the MO operation as a single-frequency laser.

The ability to reliably provide watts of surface-emitted, CWsingle-frequency, diffraction-limited power allows a wide variety ofapplications. Particularly attractive is the fact that, compared toedge-emitters, surface-emitting (SE) sources can be tested at the waferlevel, are relatively easy to package, and are not subject to facetdegradation (i.e., are more reliable). A major application involvesusing parametric frequency conversion to the mid-IR spectral range(λ=3-5 μm). Room-temperature CW mid-IR coherent light is valuable fornoninvasive medical-diagnostics techniques based on laser-absorptionspectroscopy such as breath analysis and body-fluid analysis. For breathanalysis the sensitivity may be increased by 3 orders of magnitude todetect most vital-organ malfunctions, metabolic disorders, and(invisible) traumas. In turn, doctors can use small, portable units forimmediate diagnostics in their offices or during critical times such assurgery. For body-fluid analysis such lasers may be utilized for quickand highly sensitive detection of organic-analyte (glucose, cholesterol)levels. Another application of CW room temperature mid-IR lasers is forIR countermeasures, for which the use of lightweight, high-efficiency,small sources is crucial.

The invention may also be utilized in the generation of blue light viasecond harmonic generation. The bandwidth needed varies from ≈13 Å forinefficient (≈10%) doubling crystals to ≈1 Å for highly efficient (≈50%)resonant doubling crystals. By using ROW-SE-DFB devices, hundred ofmilliwatts of blue light may be generated for use in such applicationsas laser-beam projection; high-speed, high-density optical recording;and especially biotechnology (flow cytometry, capillary electrophoresis,etc.).

For free-space optical communications to supplant RF-based technology,coherent optical communications schemes need to be implemented.ROW-SE-DFB arrays can provide the power (˜1 W), narrow-linewidth (≈1MHz), and modulation bandwidth (1-2 GHz) needed for such systems. Therewould be no need for the external phase-corrective feedback mechanismsthat are necessary if MOPA-type sources are to be used, and reliabilitywill be assured by the ROW-array intrinsic stability. Other applicationsare as sources for high-power low-noise, high-fidelity RF links,differential-absorption LIDAR, and coherent ranging over long distances.

Scaling at the wafer level can provide tens of watts of coherent power.Many uses are possible. These include significant increases in theefficiency as well as the resolution of MRI with noble gases, a novelmedical-diagnostics technique that allows high-resolution imaging of thelungs and the brain. The technique has ≈10⁶ higher resolution thanconvention MRI.

Generally, the output power available from the high-powered 2-D surfaceemitting laser array of the invention is proportional to the area of theemitting aperture. For the array devices shown in FIG. 8, a 40-elementresonant anti-guided array will have a lateral dimension of about 200μm. The longitudinal dimension corresponds to the width of the emissionfrom the second order DFB grating 22 and the DBR gratings 24, withtypical total longitudinal dimensions of about 1,500 μm. Devices of suchdimensions are capable of providing single mode output power in therange of about 3 watts. To provide higher output powers from a singlearray device 20 (as opposed to a coupled array of such devices as shownin FIGS. 15 and 16) it is necessary to increase the emitting area of thedevice, inasmuch as the output power scales approximately in relation tothe area of the aperture. For example, to obtain 50-70 watts of coherentoutput power, it would be necessary to use very long devices, in therange of 1 cm, with lateral dimensions in the range of 300-500 μm. Withvery long DFB gratings having a central discrete π phase shift, theslope efficiency decreases with the length, and the selection of thedesired in-phase array mode due to gratings placed only in the arrayelement regions becomes progressively smaller, since the DBR regions(e.g., each about 500 μm long) are much smaller than the DFB (e.g.,about 9,000 μm long). For such long structures, a grating can beutilized as illustrated in FIG. 18 that has a continuously and slowlyvarying period A that creates a distributed phase shift over the lengthof the DFB grating and DBR gratings as opposed to a discrete π phaseshift at the center of the DFB grating. Distributed phase shift gratings(which may be referred to herein as “chirped gratings”) provide surfaceemission in a single-lobed beam pattern which is slightly off normal,but stable. For such chirped gratings the slope efficiency does notdecrease with increasing device length, and thus the surface emittedpower scales with increasing grating length. Such distributed phaseshift or chirped gratings are well known and described in, e.g., U.S.Pat. Nos. 5,238,531 and 5,241,556. See also, S. H. Macomber, “NonlinearAnalysis of Surface-Emitting Distributed Feedback Lasers,” IEEE J. ofQuant. Elect., Vol. 26, No. 12, December 1990, pp 2065-2074; Steven H.Macomber, “Design of High-Power, Surface-Emitting DFB Lasers forSuppression of Filamentation,” Proc. of SPIE, Vol. 4993 (2003), pp37-49.

In relatively smaller arrays having, e.g., 20-40 laterally spacedelements, lateral edge radiation loss can be used to discriminateagainst adjacent arrays modes. However, the effect of edge loss as adiscriminator against the adjacent array modes decreases with increasingnumbers of elements. As discussed above, interelement loss alsofunctions to discriminate against the out-of-phase modes, and stronginterelement loss can be utilized to adequately suppress adjacent arraymodes to allow 60-100 element or more arrays (e. g., 300-500 μm lateraldimensions). As indicated above, highly doped GaAs as the material 55 inthe interelement regions can provide strong absorption anddiscrimination of the out-of-phase modes. Another example of ainterelement region material 55 is shown in FIG. 19, that provides verystrong absorption and discrimination, is composed of a first or baselayer 81 of GaAs of moderate doping, a second intermediate layer 82 ofGaAs that is heavily doped, and top layer 84 of the high-index materialInGaAs. The layer of high-index material 84 draws the field towards thetop surface and thus causes greater absorption in the layer 84 (thematerial of the layer 84, InGaAs, is significantly more absorbing thatGaAs since it has a lower energy band gap than GaAs) and also causesmore absorption in the metal contact above the interelement region whichis in contact with the layer 84. The in-phase mode at resonance hasabout 99 percent of its field in the element, while the other arraymodes have 70-80 percent of their field in the elements and the rest inthe interelement regions. Thus, strong light absorption in theinterelement regions suppresses the non-resonant modes while notsubstantially affecting the resonant in-phase mode.

The grating period Λ, or the phase shift of the grating from its phaseat the center of the grating, Φ_(g), can be selected to provide theappropriate second order grating feedback and first order gratingsurface emission. A general expression for the grating function isprovided below.

The grating shape is expressed in a polynomial expansion of its phase:

${\Phi_{g}(z)} = {2\;\pi\;{\sum\limits_{n = 0}^{\infty}{C_{n}\left( {2{z/L}} \right)}^{n}}}$where:

-   z is the longitudinal dimension (i.e., the direction along which the    grating phase is varying) and: z=0 is at the center of the grating,    L is the length of the whole grating, and the coefficients C_(n)    represent the number of waves of phase from the grating center to    the grating end for a given n value. For example, for a grating    linear period variation, n=2, and then a value of 0.5 for C gives a    value of π for Φ_(g)(z), which means a half-wave variation over half    the grating; that is, a full-wave variation over the whole grating    (the variation being linear).

A simple way to explain what the coefficient C stands for is thefollowing:

-   Assume that z=0 is at the center of the grating and that the    gratings's whole length is L. If this 2^(nd)-order grating is    periodic, then L=mλ, where m is an integer and λ is the wavelength    of the light in the semiconductor medium. If the grating is a    linearly chirped, as described in the above paragraph (i.e., n=2 in    the above formula) then L=(m+C₂) λ, where C₂ is a non-integer number    representing the number of waves of phase from the grating center to    the grating end. As mentioned above, if one has a full-wave    variation (i.e., 2π) over the whole grating, then C₂=0.5. For a    distributed π phaseshift over the whole grating length, C₂=0.25. If    the grating is not linearly chirped

${L = {\left( {m + {\sum\limits_{n = 0}^{\infty}C_{n}}} \right)\mspace{11mu}\lambda}},{{where}\mspace{14mu}{\sum\limits_{n = 0}^{\infty}C_{n}}}$is a non-integer number representing the number of waves of phase fromthe grating center to the grating end. The case C₀ corresponds to aphase constant which has nothing to do with the grating.

FIGS. 20-23 illustrate the results of simulations of a device as shownin FIG. 18 with a chirped grating, having a 20-element array in thelateral direction, and interelement regions which have varying amountsof interelement loss. In each of these figures, Mode 38 is the in-phasemode, and the graphs illustrate the threshold current density as afunction of interelement spacing. FIG. 20 shows the results with nointerelement loss (showing very little distinction for the thresholdcurrent between the in-phase mode and the out-of-phase and adjacentarray modes), FIG. 21 shows the affect of interelement absorption byheavily doped GaAs, FIG. 22 shows the effect of interelement absorptionin the InGaAs layer 84, and FIG. 23 shows the effect of interelementabsorption from the layer 84 and a heavily doped GaAs layer 82. Asillustrated in FIG. 23, with the use of the InGaAs layer 84 and heavilydoped GaAs, at an interelement spacing in the range of about 1.07 to1.11 μm only the in-phase mode 38 is the one favored to lase since ithas much lower threshold current density than the other modes. Thesimulations are shown for a DBR/DFB/DBR structure (in μm) inlongitudinal direction of 700/8,000/700, with the following materials:400 nm-thick InGaP upper cladding layer; grating depth: 100 nm, 50nm-thick InGaAs, 220 nm-thick GaAs, InGaAs absorption coefficient valueof 8,000 cm⁻¹ and index value of n=3.7; the upper 110 nm of the GaAsregrown material with absorption coefficient of 600 cm⁻¹, the lower 110nm of the GaAs material with absorption coefficient of 10 cm⁻¹.

A phase shift to obtain a single lobed output can also be obtained asillustrated in FIG. 24 by utilizing a phase shifting film 90 on theoutput surface 41 of the substrate to provide a single lobed output beam91. A DFB grating 22 may then be utilized which has a regular periodicgrating with a constant period Λ. Such surface films to obtain π phaseshifting are described in U.S. Pat. No. 4,805,176 to Botez, et al.,entitled “Phase-Locked Laser Array with Phase-Shifting Surface Coating,”the disclosure of which is incorporated herein by reference. See also,D. E. Ackley, et al., “Phase-Locked Injection Laser Arrays withIntegrated Phase Shifters,” RCA Review, Vol. 44, December 1983, pp625-633; M. Matsumoto, et al., “Single-Lobed Far-Field Pattern Operationin a Phased Array with an Integrated Phase Shifter,” Appl. Phys. Lett.,Vol. 50, No. 22, 1 June 1987, pp. 1541-1543; and S. H. Macomber, et al.,“Recent Developments in Surface Emitting Distributed Feedback Arrays,”Proc. SPIE, Vol. 1219 Laser-Diode Technology and Applications II, 1990,pp. 228-232. The phase shifting film 90 is deposited on the outputsurface above about half of the length of the grating, which serves toshift the beam phase by π. The external, emitted beam 91 thus becomessingle lobed rather than the double lobed internal beam.

It is understood that the invention is not limited to the embodimentsset forth herein as illustrative, but embraces all such forms thereof ascome within the scope of the following claims.

1. A semiconductor laser comprising: (a) a semiconductor substrate; (b) an array of laterally-spaced laser device elements formed on the substrate, each of the laser device elements extending in a longitudinal direction on the substrate, each laser device element including a second order distributed feedback grating having optical gain and each laser device element having distributed Bragg reflector gratings bounding the distributed feedback grating in the longitudinal direction to reflect light back to the distributed feedback grating, wherein the gratings are confined to the laser device elements; (c) electrodes by which voltage can be applied across the array and the substrate, and means for directing current to a region of the array containing the distributed feedback gratings of the laser device elements; and (d) laser device interelement regions on the substrate between the device elements, the device elements in which the distributed feedback grating and the distributed Bragg reflector gratings are formed having a lower effective refractive index than the index of the interelement regions, the interelement regions spacing the device elements by a width such that light propagating laterally from the device elements is fully transmitted between adjacent device elements to form a laterally resonant antiguided array for which the device elements are substantially equally coupled to each other, the interelement regions having strong light absorption to provide discrimination of modes with significant interelement field.
 2. The semiconductor laser of claim 1 wherein the laser device elements and the laser device interelement regions are formed in an epitaxial structure on the substrate including a layer with an active region at which light emission occurs, upper and lower cladding layers surrounding the active region layer, each distributed feedback, grating in the device elements incorporated with the epitaxial structure having a selected phase shift and comprising periodically alternating grating elements to provide optical feedback as a second order grating for a selected effective wavelength of light generation from the active region, the grating having a spacing between adjacent grating elements at a selected position intermediate that corresponds to the selected phase shift in the grating, the grating formed and positioned to act upon the light generated in the active region to produce a lasing action and an emission of light from a lower face of the, substrate of the semiconductor laser, and wherein the distributed Bragg reflector gratings for each laser device element are incorporated with the epitaxial structure adjacent the distributed feedback grating in each laser device element to reflect light back to the distributed feedback grating.
 3. The semiconductor laser of claim 2 wherein the electrodes are formed on an upper face and a lower face of the semiconductor laser and for the electrode formed on the upper face current flows through the region that contains the distributed feedback gratings.
 4. The semiconductor laser of claim 2 wherein the active region layer is formed of InGaAsP confinement layers and at least one InGaAs quantum well layer between the InGaAsP confinement layers, and the lower and upper cladding layers are formed of n-type InGaP and p-type InGaP, respectively, and the substrate is formed of GaAs.
 5. The semiconductor laser of claim 2 wherein the distributed Bragg reflector gratings of each laser device element and the laser device interelement regions have longitudinal ends and including a light absorbing material formed on the structure at the longitudinal ends of the distributed Bragg reflector gratings and interelement regions to absorb light that passes out from the distributed Bragg reflector gratings and the interelement regions to prevent reflections of light back into the laser device.
 6. The semiconductor laser of claim 1 wherein the means for directing current flow to the distributed feedback gratings include an insulating layer on the distributed Bragg reflector gratings to inhibit current flow through these gratings.
 7. The semiconductor laser of claim 1 wherein one of the electrodes is formed on a lower face of the substrate and has a window opening formed therein to permit light emission therethrough.
 8. The semiconductor laser of claim 2 wherein the distributed feedback grating has a central phase shift in the grating within 30° of 180°.
 9. The semiconductor laser of claim 1 wherein the interelement regions are formed of highly doped GaAs.
 10. The semiconductor laser of claim 1 wherein the interelement regions include a first layer of GaAs of a selected doping type, a second layer of GaAs more heavily doped than the first layer and of the same selected doping type as the first layer, and a top layer of InGaAs.
 11. The semiconductor laser of claim 1 wherein the array laterally-spaced device elements have a total lateral dimension of at least 250 μm.
 12. The semiconductor laser of claim 1, wherein each second order distributed feedback grating has a chirped period that corresponds to a phase shift distributed across the grating, and the distributed Bragg reflector gratings also have chirped periods which correspond to distributed phase shifts.
 13. The semiconductor laser-of claim 12 wherein the laser device elements and the laser device interelement regions are formed in an epitaxial structure on the substrate including a layer with an active region at which light emission occurs, upper and lower cladding layers surrounding the active region layer, each distributed feedback grating in the device elements incorporated with the epitaxial structure comprising periodically alternating grating elements to provide optical feedback as a second order grating for a selected effective wavelength of light, generation from the active region, the grating having spacings between adjacent grating elements that correspond to the selected chirping function of the grating period, the grating formed and positioned to act upon the light generated in the active region to produce a lasing action and an emission of light from a lower face of the substrate of the semiconductor laser, and wherein the distributed Bragg reflector gratings for each laser device element are incorporated with the epitaxial structure adjacent the distributed feedback grating in each laser device element to reflect light back to the distributed feedback grating and which also have spacings between adjacent grating elements that correspond to the selected chirping function of the grating period.
 14. The semiconductor laser of claim 13 wherein the electrodes are formed on an upper face and a lower face of the semiconductor laser and for the electrode formed on the upper face current, flows through the region that contains the distributed feedback gratings.
 15. The semiconductor laser of claim 13 wherein the active region layer is formed of lnGaAsP confinement layers and at least one InGaAs quantum well layer between the InGaAsP confinement layers, and the lower and upper cladding layers are formed of n-type InGaP and p-type InGaP, respectively, and the substrate is formed of GaAs.
 16. The semiconductor laser of claim 13 wherein the distributed Bragg reflector gratings of each laser device element and the laser device interelement regions have longitudinal ends and including a light absorbing material formed on the structure at the longitudinal ends of the distributed Bragg reflector gratings and interelement regions to absorb light that passes out from the distributed Bragg reflector gratings and the interelement regions to prevent reflections of light back into the laser device.
 17. The semiconductor laser of claim 12 wherein the means for directing current flow to the distributed feedback gratings include an insulating layer on the distributed Bragg reflector gratings to inhibit current flow through these gratings.
 18. The semiconductor laser of claim 12 wherein one of the electrodes is formed on a lower face of the substrate and has a window opening formed therein to permit light emission therethrough.
 19. The semiconductor laser of claim 12 wherein the distributed phase shift across the length of all gratings totals about 180°.
 20. The semiconductor laser of claim 12 wherein the interelement regions are formed of highly doped GaAs.
 21. The semiconductor laser of claim 12 wherein the interelement regions include a first layer of GaAs of a selected doping type, a second layer of GaAs more heavily doped than the first layer and of the same selected doping type as the first layer, and a top layer of InGaAs.
 22. The semiconductor laser of claim 12 wherein the array laterally spaced device elements have a total lateral dimension of at least 300 μm.
 23. The semiconductor laser of claim 1, further comprising: (e) a π phase shifting film formed on the substrate at an emission window of the substrate by which light is emitted covering about one half of the emission window to provide a π phase shift and substantially single lobe emission from the emission window of the semiconductor substrate.
 24. The semiconductor laser of claim 23 wherein the laser device elements and the laser device interelemnt regions are formed in an epitaxial structure on the substrate including a layer with an active region at which light emission occurs, upper and lower cladding layers surrounding the active region layer, each distributed feedback grating in the device, elements incorporated with the epitaxial structure comprising periodically alternating grating elements to provide optical feedback as a second order grating for a selected effective wavelength of light generation from the active region the grating formed and positioned to act upon the light generated in the active region to produce a lasing action and an emission of light from a lower face of the substrate of the semiconductor laser, and wherein the distributed Bragg reflector gratings for each laser device element are incorporated with the epitaxial structure adjacent the distributed feedback grating in each laser device element to reflect light back to the distributed feedback grating.
 25. The semiconductor laser of claim 24 wherein the electrodes are formed on an upper face and a lower face of the semiconductor laser and for the electrode formed on the upper face current flows through the region that contains the distributed feedback gratings.
 26. The semiconductor laser of claim 24 wherein the active region layer is formed of InGaAsP confinement layers and at least one InGaAs quantum well layer between the LnGaAsP confinement layers, and the lower and upper cladding layers are formed of n-type InGaP and p-type InGaP, respectively, and the substrate is formed of GaAs.
 27. The semiconductor laser of claim 24 wherein the distributed Bragg reflector gratings of each laser device element and the laser device interelement regions have longitudinal ends, and including a light absorbing material formed on the structure at the longitudinal ends of the distributed Bragg reflector gratings and interelement regions to absorb light that passes out from the distributed Bragg reflector gratings and the interelement regions to prevent reflections of light back into the laser device.
 28. The semiconductor laser of claim 23 wherein the means for directing current flow to the distributed feedback gratings include an insulating layer on the distributed Bragg reflector gratings to inhibit current flow through these gratings.
 29. The semiconductor laser of claim 23 wherein one of the electrodes is formed on a lower face of the substrate and has a window opening formed therein to permit light emission therethrough.
 30. The semiconductor laser of claim 23 wherein the interelement regions are formed of highly doped GaAs.
 31. The semiconductor laser of claim 23 wherein the interelement regions include a first layer of GaAs of a selected doping type, a second layer of GaAs more heavily doped than the first, layer and of the same selected doping type as the first layer, and a top layer of InGaAs.
 32. The semiconductor laser of claim 23 wherein the distributed Bragg reflector gratings longitudinally bounding the distributed feedback grating in each laser device element vary in length as to cause the portions of the laser device containing distributed Bragg reflectors to taper longitudinally as to be narrower in width at outer ends of the laser than at the position where the distributed Bragg reflectors meet the distributed feedback gratings.
 33. The semiconductor laser of claim 32 wherein the distributed Bragg reflector gratings of laser device elements and laser device interelement regions have longitudinal ends at the laser outer ends and including a light absorbing material formed on the structure at the longitudinal ends of the distributed Bragg reflector gratings and interelement regions to absorb light that passes out from the distributed Bragg reflector gratings and the interelement regions to prevent reflections of light back into the laser device.
 34. The semiconductor laser of claim 32 wherein a light absorbing material formed on the structure has angled edges in the regions where the distributed Bragg reflector gratings do not reach the laser outer ends to prevent reflections of light back into the laser device.
 35. The semiconductor laser of claim 1 wherein the laser device elements and the laser device interelement regions are formed in an epitaxial structure on the substrate including a layer with an active region at which light emission occurs, upper and lower cladding layers surrounding the active region layer, each distributed feedback grating in the device elements incorporated with the epitaxial structure having a selected phase shift and comprising periodically alternating grating elements to provide optical feedback as a second order grating for a selected effective wavelength of light generation from the active region, the grating having spacings between adjacent grating element at a selected position intermediate that correspond to the selected phase shift in the grating, the grating formed and positioned to act upon the light generated in the active region to produce a lasing action and an emission, of light from a lower face of the substrate of the semiconductor laser, and wherein the distributed Bragg reflector gratings for each laser device element are incorporated with the epitaxial structure adjacent the distributed feedback grating in each laser device element to reflect light back to the distributed feedback grating. 